Jul 23 Sunday 2017

Tropical Report

Tropical Weather Outlook
Tropical Weather Outlook
NWS National Hurricane Center Miami FL
200 AM EDT Sun Jul 23 2017

For the North Atlantic…Caribbean Sea and the Gulf of Mexico:

Tropical cyclone formation is not expected during the next 5 days.

$$
Forecaster Zelinsky

Tropical Storm Hilary Discussion Number 8
NWS National Hurricane Center Miami FL EP092017
400 AM CDT Sun Jul 23 2017

The cloud pattern of Hilary is gradually becoming better organized,
with some convective banding features beginning to develop. The
upper-level outflow is well defined over all but the southeastern
semicircle of the circulation. Dvorak T-numbers from TAFB and SAB
remain at 2.5 so the current intensity is held at 35 kt. This is
also consistent with ASCAT data from a few hours ago. The
atmospheric and oceanic environment should be quite favorable for
intensification during the next few days. The SHIPS guidance
indicates that the vertical shear should remain below 10 kt
throughout the forecast period, and the global models show a large
upper-level anticyclone over Hilary through 4-5 days. Late in the
period Hilary is expected to move over cooler SSTs, and this should
halt the intensification process. The official intensity forecast
shows the system approaching major hurricane status in 72 hours
and some of the guidance, including the corrected consensus models,
suggest that this could be conservative.

Overnight microwave data indicate a bit of a jump to the
west-northwest, perhaps as a result of reformation of the center as
opposed to large-scale motion. The initial motion estimate is
300/13 kt. A large mid-level anticyclone centered over the
southwestern United States or northwestern Mexico should maintain
the west-northwestward heading throughout the forecast period. The
track model guidance is in agreement on a decrease in forward speed,
and the official forecast shows this as well. Later in the period,
there is a possibility of some binary interaction with Tropical
Storm Irwin currently located some 800 n mi to the west. This, along
with a substantial spread in the track models at 4-5 days, adds some
uncertainty to the forecast. In any event, the official track
forecast is not far from the latest model consensus, and keeps the
system well offshore of the coast of Mexico.

FORECAST POSITIONS AND MAX WINDS

INIT 23/0900Z 12.5N 101.6W 35 KT 40 MPH
12H 23/1800Z 13.1N 102.8W 45 KT 50 MPH
24H 24/0600Z 13.9N 104.3W 55 KT 65 MPH
36H 24/1800Z 14.7N 105.8W 65 KT 75 MPH
48H 25/0600Z 15.5N 107.3W 80 KT 90 MPH
72H 26/0600Z 17.0N 111.0W 95 KT 110 MPH
96H 27/0600Z 18.0N 115.0W 95 KT 110 MPH
120H 28/0600Z 19.0N 118.5W 85 KT 100 MPH

$$
Forecaster Pasch

Tropical Storm Irwin Discussion Number 4
NWS National Hurricane Center Miami FL EP102017
200 AM PDT Sun Jul 23 2017

An ASCAT-B overpass at 0516 UTC showed several vectors of 32-33 kt
winds and a max value of 34 kt in the northeast quadrant of the
tropical cyclone. Based on that data, the system has been named
Tropical Storm Irwin. Irwin is the fifth system to be named in the
eastern North Pacific so far in July.

Although the system has been upgraded based on the ASCAT data, the
structure has not improved. Shortwave-IR imagery shows that the
low-level center is clearly exposed to the northwest of the deep
convection, and Dvorak intensity estimates from TAFB and SAB have
not increased. The global models suggest that the shear should
decrease quite a bit after about 12 to 24 hours, which should allow
for some intensification. The regional dynamical models HWRF and
COAMPS-TC depict Irwin quickly recovering from its current sheared
state and intensifying to a category 2 or 3 hurricane. On the other
hand, the statistical models still only show modest strengthening,
and keep Irwin below hurricane strength. As a course of least
regret, the intensity forecast splits these two scenarios, but
slightly favors the lower statistical guidance since the dynamical
models may show the structure of Irwin recovering from the current
shear too quickly. Near the end of the forecast period, Irwin is
expected to encounter a higher shear environment, due in part to
outflow from Hilary to the east, so the forecast depicts gradual
weakening.

The initial motion is 280/6 kt. Irwin continues to be steered by a
low- to mid-level ridge to the northwest. Although no significant
changes have been made to the track forecast, confidence is still
somewhat low since there is a fair amount of speed-spread in the
models. The GFS remains an outlier in being the only model that
shows substantial binary interaction with Hilary, and is thus the
farthest east, though the 00 UTC ECMWF is also a little slower
than before. The official forecast is a little slower at the end of
forecast period than the previous advisory, but still assumes that
any significant track deviation due to interactions with Hilary
will occur beyond 120 h.

FORECAST POSITIONS AND MAX WINDS

INIT 23/0900Z 14.7N 115.2W 35 KT 40 MPH
12H 23/1800Z 14.8N 116.0W 40 KT 45 MPH
24H 24/0600Z 14.9N 117.0W 45 KT 50 MPH
36H 24/1800Z 15.0N 118.0W 50 KT 60 MPH
48H 25/0600Z 15.0N 118.8W 60 KT 70 MPH
72H 26/0600Z 14.7N 121.0W 65 KT 75 MPH
96H 27/0600Z 14.5N 123.1W 60 KT 70 MPH
120H 28/0600Z 14.0N 125.5W 55 KT 65 MPH

$$
Forecaster Zelinsky

Tropical Storm Greg Discussion Number 24
NWS National Hurricane Center Miami FL EP072017
200 AM PDT Sun Jul 23 2017

The center of Greg has been difficult to locate tonight, but earlier
microwave data and an ASCAT pass that just barely caught the center
indicate that the tropical storm remains sheared. At the time of
those passes, the center was located just southeast of the main
convective mass and there is no reason to believe that has changed.
Although the ASCAT pass missed the western portion of the
circulation, it still showed winds of 38 kt in the northeast
quadrant. Assuming the winds are a little higher within the
convection, the initial intensity has been held at 45 kt.

Although Greg is moving away from an upper-level low and the worst
of the vertical shear, the near-storm environment is still fairly
dry, as diagnosed in the GFS and ECMWF-based SHIPS. This should
be enough to prevent the cyclone from strengthening, and very
gradual weakening is supported by most of the guidance. After
about 48 hours, cooler SSTs and increasing shear should result in
additional weakening, and Greg is forecast to become a remnant low
by day 4. The new intensity forecast is very similar to the
previous advisory.

The aforementioned ASCAT pass suggested that the center of Greg is a
little farther south than previously estimated, and the track
forecast has been adjusted accordingly. Aside from that, little
change has been made and the official forecast is near the middle
of the tightly clustered guidance. All of the global models still
show Greg moving along the southern periphery of a low- to mid-level
ridge for the next couple of days, followed by a turn toward the
northwest as the ridge weakens slightly. By the end of the
forecast period a turn back to the west as a remnant low is
expected, when Greg becomes steered primarily by low-level trade
winds.

FORECAST POSITIONS AND MAX WINDS

INIT 23/0900Z 14.7N 127.7W 45 KT 50 MPH
12H 23/1800Z 14.6N 129.5W 45 KT 50 MPH
24H 24/0600Z 14.6N 131.9W 40 KT 45 MPH
36H 24/1800Z 14.8N 133.9W 40 KT 45 MPH
48H 25/0600Z 15.2N 135.5W 35 KT 40 MPH
72H 26/0600Z 16.1N 138.0W 30 KT 35 MPH
96H 27/0600Z 16.9N 140.4W 25 KT 30 MPH…POST-TROP/REMNT LOW
120H 28/0600Z 17.0N 143.5W 25 KT 30 MPH…POST-TROP/REMNT LOW

$$
Forecaster Zelinsky